Interactions between Volatiles and Char during Pyrolysis of Biomass

Jul 5, 2016 - HGMCN interacted with LG only at 400 °C, while the LG–CGMCN interactions started at 400 °C and intensified at 450 °C with a signifi...
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Interactions between Volatiles and Char during Pyrolysis of Biomass: Reactive Species Determining and Reaction over Functionalized Carbon Nanotubes Yong Huang,† Yalun Hu,† Jun-ichiro Hayashi,*,‡,§,∥ and Yunming Fang*,† †

Department of Chemical Engineering, Beijing University of Chemical Technology, 100029, Beijing, China Institute for Materials Chemistry and Engineering, Kyushu University, 6-10, Kasuga Koen, Kasuga 816-8580, Japan § Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-10, Kasuga Koen, Kasuga 816-8580, Japan ∥ Research and Education Center of Carbon Resources, Kyushu University, 6-10, Kasuga Koen, Kasuga 816-8580, Japan ‡

S Supporting Information *

ABSTRACT: This paper investigated the interactions between volatiles and char during pyrolysis of biomass with an emphasis on those between the reactive compounds in volatiles and the oxygen-containing functional groups in char. On the basis of a comparative study on the products from biomass pyrolysis with and without heavy oil (HO) recycling, which represented a maximization and minimization of the volatile−char interactions, respectively, levoglucosan (LG) was selected as the typical reactive compound in the volatiles. The interactions between LG and functionalized graphitized multiwalled carbon nanotubes with hydroxyl and carboxyl groups (HGMCN and CGMCN, respectively) as char models were surveyed at 300, 350, 400, and 450 °C. The results show that temperature plays an important role in the interactions between LG and GMCNs. HGMCN interacted with LG only at 400 °C, while the LG−CGMCN interactions started at 400 °C and intensified at 450 °C with a significant increase in gas yield. The GC−MS analyses indicated that the liquid products from the LG−GMCNs interactions were predominant in furfural and 5-methyl-2-furancarboxaldehyde.

1. INTRODUCTION Biomass, the sole carbon-containing renewable source, has become one of the most popular research objects with an increasing demand for liquid fuel and decreasing reserve of fossil fuels.1,2 Pyrolysis is the simplest thermochemical method to convert biomass to char and bio-oil, whose yields are easily adjusted by the reaction conditions, e.g., the heating rate and peak temperature.3−5 It is reported that the yield of bio-oil can be as high as 70 wt % of dry feedstock from a flash pyrolysis.6,7 The bio-oil with such high yield has so far been, however, used only as an alternative to petroleum-derived fuel oil to be burned for heat/steam generation.8−10 This is mainly due to the high content of oxygen in bio-oil, which is commonly present in reactive polar compounds, such as hydroxyaldehydes, hydroxyketones, sugars, carboxylic acids, and phenols. Some of them have propensities to undergo polymerization during the reheating, resulting in that 35−50 wt % of the starting bio-oil is often left as solid residue or coke.11,12 Thus, the focus of biomass research is gradually changing from maximization of bio-oil yield to optimization of its quality.13 Up to now, many researches have been conducted to improve the quality of bio-oil. Catalytic hydrodeoxygenation is one of the most used and effective methods for upgrading biooil. Vispute et al.14 employed a two-step method for conversion of bio-oil to light olefins and aromatic hydrocarbons using Ru/ C and/or Pt/C, followed by ZSM-5 as catalysts. The results showed that the first step of hydrodeoxygenation not only significantly inhibited the coke formation on the ZSM-5 surface used in the second step but also increased the yield of desired products. Marker et al.15,16 integrated hydropyrolysis and © XXXX American Chemical Society

hydroconversion processes to yield gasoline and diesel from biomass. The hydropyrolysis step reduced the oxygen content to 2.7% and thus minimized the coke formation in the hydroconversion process, resulting in the catalyst life being as long as 750 h. Mercader et al.17 coprocessed a hydrodeoxygenated pyrolysis bio-oil with a Long Residue as a diluent and hydrogen transfer source to generate gasoline and light cycle oil, which demonstrated that the hydrodeoxygenation of bio-oil, although not completely, played an important role in suppression of the formation of coke and dry gas. However, hydrodeoxygenation is usually performed under strict conditions, e.g., high pressure, the presence of catalyst and hydrogen.18 In our previous studies,19,20 a novel countercurrent moving bed reactor (MB), which consisted of a capturing zone (CZ) at the upper part and pyrolysis zone (PZ) at the lower part, was proposed to simultaneously maximize the char yield and volatility of oil. In the proposed system, a heavier portion of oil (HO) was internally recycled between CZ and PZ by feedstock capture and repyrolysis, respectively, while lighter portion of oil (LO) and gas escaped from the top of the CZ. The HO was thus converted to char, LO, and gas within the reactor during its recycling without either catalyst or high hydrogen pressure. The resulting LO from a pine biomass was completely evaporated at 220 °C, and its char yield increased by a factor of 1.44, resulting from both self-charring of the captured HO Received: March 29, 2016 Revised: June 17, 2016

A

DOI: 10.1021/acs.energyfuels.6b00725 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

°C for 90 min, the container train moved up and passed through an upper nonisothermal zone (length, 425 mm) with a temperature gradient ranging from 120 to 550 °C. When the bottom of CT10 reached the top of the upper nonisothermal zone, the movement of the container train was stopped and CT1−CT10 was removed to finish the first run. The second run was conducted starting from the initial lower isothermal zone with a new train of CT11−CT32, of which CT11−CT22 was recycled from the first run and CT23−CT32 was filled with fresh feedstock. In the same manner, three runs were performed. BO1 was a combination of the liquid products from two condensers in the third run. BO2 was collected from a horizontal tubular reactor with a large flow rate of N2, e.g., 1000 mL/min, in which a thin layer of around 3 g of pine was placed. The large flow rate and thin sample bed minimized the interactions between volatiles and char. The product recovery was the same as that used for producing BO1. The heating rate and peak temperature for the two pyrolyses were similar, as shown in Figure S1. 2.2. Reaction between LG and GMCNs. Hydroxylate/carboxyl/ nonfunctionalized GMCNs (denoted as HGMCN/CGMCN/ NFGMCN; outer diameter, 8−15 nm; length, ∼50 μm; purity, >99.9%) were purchased from Nanjing XFNANO Materials Tech Co., Ltd., China. GMCNs were prepared by a catalytic chemical vapor deposition method, and HGMCN with 1.85 wt % of hydroxyl groups and CGMCN with 1.28 wt % of carboxyl groups were oxidized from the original GMCNs by the manufacturer. Figure 1 exhibits a

and chemical interactions between volatiles and pyrolyzing char. In fact, the interactions between bio-oil volatiles and char are inevitably present in all biomass pyrolysis reactors as long as more than one feedstock particle is pyrolyzing. Some studies on the volatile−char interactions have been conducted, most of which concentrated on the effect of the volatile−char interactions on the volatilization of alkali and alkaline earth metallic (AAEM) species21−24 and the evolution of the char structure.25−27 However, the effect of volatile−char interactions on bio-oil decomposition and which composition in volatile and char attributes to such volatile−char interactions are still not clear. Cellulose-derived reactive sugar monomers and oligomers in volatiles28,29 and reactive functional groups in char, e.g., oxygen-containing groups,27,30 are considered to play key roles in the volatile−char interactions. Examination of the reaction between them is thus necessary to better understand the volatile−char interactions. Unfortunately, the reactive functional groups are only present in the char prepared at low temperature.30,31 On the other hand, low-temperature char is characterized by thermal instability and volatile emission, which makes the understanding of the volatile−char interactions extremely difficult. A graphitized multiwalled carbon nanotube (GMCN) is featured by its high thermal and chemical stabilities and volatile-free property.32 A functionalized GMCN (FGMCN) with different types of functional groups is thus an advisible model for low-temperature char. In continuation of the previous studies, this paper carefully compared the changes in two different bio-oils, which were produced from MB with HO recycling (BO1) and a fixed-bed pyrolysis (FB) without HO recycling (BO2). Such a comparative study is beneficial to find the reactive compounds that easily react with char during HO recycling. On the other hand, the interactions between the reactive compounds and char may be influenced by the volatiles residence time in the reactor and/or the presence of functional groups in char. It is thus important to verify and distinguish these two events. Although AAEM species in char also affect the volatile−char interactions to a certain degree,33,34 considering that the feedstock employed for the production of BO1 and BO2 contains an insignificant ash content, the effect of AAEM species will not be studied here. In this study, the composition of BO1 and BO2 was analyzed by gas chromatography/mass spectrometry (GC/MS) to confirm the reactive compounds interacted with char during the HO recycling. The interactions between levoglucosan (LG) and FGMCNs as the models of reactive bio-oil and lowtemperature char, respectively, were also systematically investigated.

2. EXPERIMENTAL SECTION 2.1. Production of BO1 and BO2. The feedstock (pine) and the procedure for the production of BO1 and BO2 were kept consistent as those employed for the previous studies.19,35 In the case of BO1, dried pine (water content, ∼2 wt %) was filled into a vertical reactor, which consisted of a train of 22 cylindrical containers (top to bottom, CT1 to CT22) with an inner diameter of 25 mm and a depth of 50 mm for each. Every container held about 7 g of pine and was isolated from the upper/lower containers by a wire-mesh sheet (mesh opening, 180 μm) as a vapor−solid separator. Carrier gas, N2, was introduced at a rate of 200 mL/min (at 20 °C) from the top of the uppermost container. The outlet at the bottom of the lowermost container was connected to an aerosol filter, two condensers at 200, 0, and −70 °C, respectively. After being preheated in a lower isothermal zone (length, 1100 mm) at 120

Figure 1. Schematic diagram of the two-stage reactor for the reaction between LG and GMCNs. schematic diagram of a vertical two-stage reactor, which was employed for investigating the reaction between LG and GMCNs. In a typical run, 2 g of GMCNs and 2 g of LG wrapped by a square wire-mesh sheet (mesh opening, 180 μm) were fed into the lower and upper parts of a quartz tube (inner diameter, 2 mm), respectively. Some amount of quartz wool was used to support GMCNs, while LG was held by the wire mesh embedded in the quartz tube. N2 gas (purity of >99.9995 vol %) flowed continuously and downward at a flow rate of 200 mL/min (at 20 °C) through the quartz tube. In the downstream B

DOI: 10.1021/acs.energyfuels.6b00725 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels of the quartz tube, the trap 1 (−70 °C), trap 2 (−70 °C), and gasbag were connected in series. Using two traps at −70 °C ensured the complete recovery of organic compounds except for C 1 −C 4 hydrocarbons. After the temperature of GMCNs (inside of the quartz tube) reached 300, 350, 400, or 450 °C (controlled by Furnace 2) and held at each temperature for 90 min, Furnace 1 started to heat until the temperature of LG was achieved at 200 °C, at which LG completely evaporated within 155 min and the self-decomposition was minimized.36 Blank experiments were also conducted without using GMCNs under the same conditions as those mentioned above. 2.3. Product Analyses and Characterization. The masses of solid and liquid products for all runs were weighed to calculate their yields, while the mass of gaseous products collected in the gasbag was calculated based on the quantitative analysis of gas chromatography (GC). Considering that the third run of the proposed reactor with HO recycling reached a steady state, the product yield from MB was given by the following equation:19

equal to the yield of the recycled HO. The other was that BO1 already included secondary LO converted from the recycled HO, and the change in bio-oil yield of 12 wt % was a combination of the decrease in the recycled HO and the increase in the secondary LO. The comparison for the composition of BO1 and BO2 (showed later) supported the latter hypothesis, which indicated that the recycled HO was converted to char, gas, and also LO. 3.2. Comparative Analyses of BO1 and BO2. BO1 and BO2 were further compared by qualitative and quantitative GC/MS analyses. The main identified compounds are shown in Table 2. In summary, the changes in bio-oil resulting from the HO recycling were classified into four categories: First, the new formed compounds (nos. 1−20 with an average molecular weight of 106.8); second, the compounds with increased contents (nos. 21−45 with an average molecular weight of 103.1); third, the compounds with reduced contents (nos. 46− 54 with an average molecular weight of 121.6); fourth, the removed compounds (nos. 55−71 with an average molecular weight of 143.4). It was easily found that the average molecular weights of the compounds in Categories 1 and 2 were less than those in Categories 3 and 4. In detail, most of the monoaromatic hydrocarbons, furans, and monooxygen-containing phenols (e.g., benzene, furfural, and phenol) were newly generated or increased in their contents because of the HO recycling, while sugars and trioxygen-containing phenols (e.g., LG and 1-(4-hydroxy-3-methoxyphenyl)ethanone) were absent from BO1. Particularly, LG was considered to have played the most important role in the HO recycling because of its highest content in BO2 with 0.18 g/g bio-oil. The GC/MS results also demonstrated that the recycled HO was converted to not only char and gas but also LO, which has been discussed in section 3.1. The volatilities of BO1 and BO2 were also determined with TGA. As exhibited in Figure 2, both BO1-O and BO1-W were evaporated completely upon 220 °C, leaving no or negligible residue, while around 4 wt % of residue was left in BO2 even with the temperature as high as 900 °C. Such undesired residue generally leads to carbon deposition on the catalyst surface in the downstream upgrading of bio-oil, and its precursors are fully decomposed into char, LO, and gas during the HO recycling without high hydrogen pressure and using catalyst. Since 4 wt % of residue derived from the self-charring of BO2 is much less than the increase in char yield caused by HO recycling (11 wt %, see Table 1), the volatile−char interactions are considered to play a more significant role than self-charring. More importantly, the volatility of BO2 was higher than that of BO1 when the temperature was below 150 °C, which indicated that not only HO but also a portion of LO was converted during the HO recycling. This is in good agreement with our previous study.20 3.3. Reaction Behavior between LG and GMCNs. On the basis of the comparison for GC/MS analyses of BO1 and BO2, LG was the key compound which interacted with char during the HO recycling because of its significant change in yield. Ronsse et al.34 investigated the secondary reactions of LG and char prepared at 500 °C in a fast pyrolysis of cellulose and found that metals rather than the fixed carbon in the char catalyzed the decomposition of the LG vapor. However, the temperature for char preparation at 500 °C was high enough to remove reactive functional groups in char, e.g., hydroxyl and carbonyl groups.30,31 Indeed, the pyrolyzing biomass solid was rich in various functional groups below 500 °C.30,31 As

product yield = mass of product from the third run /total mass of products from the third run The liquid products were dissolved in acetone and subjected to gas chromatography/mass spectrometry (GC/MS) with a gas chromatograph (PerkinElmer, Inc., Clarus 680 GC) coupled to a mass spectrometer (PerkinElmer, Inc., Clarus SQ8S MS) in an electron impact mode at an ionization energy of 70 eV and with a scan range of m/z 30−550. A commercial column (GL Science Co., TC-1701; 60 m × 0.25 mm, with a film thickness of 0.25 μm) was employed for separation of compounds with the following temperature history: temperature holding at 40 °C for 5 min, heating to 280 °C at a rate of 2 °C/min, and temperature holding at 280 °C for 20 min. The interpretation of the mass spectra was primarily based on an automatic library search (NIST08, version 2.0f). The quantification method can be found in our previous publication.20 The original BO1 consisted of two phases: a water phase (BO1-W) and an oil phase (BO1-O), while BO2 was composed of only one phase. BO1-W, BO1-O, and BO2 were subjected to thermogravimetric analysis (TGA) at a heating rate of 10 °C/min from 25 to 900 °C under an atmospheric flow of N2 (100 N mL/min). Both FGMCNs before and after the experiment at 400 °C were also determined by TGA at a heating rate of 10 °C/min from 35 to 800 °C under an atmospheric flow of air (100 N mL/min).

3. RESULTS AND DISCUSSION 3.1. Product Distributions from MB and FB. Table 1 reports the product yields from MB and FB and the differences Table 1. Product Yields from MB and FB yield, wt % of dry feedstock product

FB

MB

change

char bio-oil gas

25 59 16

36 47 17

+11 −12 +1

between them. The char yield from FB, 25 wt %, was in good agreement with the char yield of CT1 from MB in the previous study,19 while an increased char yield of 36 wt % was found after three sequential runs from MB because of the HO recycling. The increase in char yield was approximately equivalent to the decrease in bio-oil yield because of an insignificant change in gas yields, i.e., 1 wt %. However, this did not necessarily mean that the recycled HO was only converted to char and gas. There were two different possibilities for the BO1 production from MB: One was that BO1 with 47 wt % was only composed of primary LO produced from the parent biomass, and the decrease in bio-oil yield with 12 wt % was C

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Energy & Fuels Table 2. Composition of BO1 and BO2 content, g/g bio-oil no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

compound 1,2-ethanediol, diacetate phenol, 2-methoxy-4-(1-propenyl)-, (E)2,5-hexanedione ethanone, 1-(2-furanyl)methyl vinyl ketone 3-furaldehyde furan, 3-methylmethyl propionate benzene, 1,3-dimethyl1,3-cyclopentadiene, 1-methylfuran, 2,4-dimethylbenzene furan, 2-ethyl-5-methyl1,3-cyclopentadiene, 5-methylfuran, 2,3,5-trimethylbenzene, 1-ethyl-3-methylethylbenzene 1-propanone, 1-(2-furanyl)benzene, 1,2,3-trimethyllimonene furfural, 5-methyl3-penten-2-one furfural propanoic acid, ethenyl ester phenol phenol, 4-methylphenol, 3-methyl1,2-cyclopentanedione, 3-methylphenol, 2-methylfuran, 2,5-dimethylphenol, 2-methoxy2(3H)-furanone, 5-methylphenol, 2,4-dimethyl2-butanone, 1-(acetyloxy)phenol, 2-methoxy-4-methylacetic acid p-xylene toluene 2(5H)-furanone, 5-methyl2-cyclopenten-1-one, 3-methyl2(5H)-furanone 1-hydroxy-2-butanone 2-propanone, 1-hydroxyphenol, 2-methoxy-4-propylbutyrolactone 2,3-butanedione 4(1H)-pyrimidinone, 6-methylphenol, 4-ethyl-2-methoxy2-cyclopenten-1-one, 2-methyl2-methyliminoperhydro-1,3-oxazine 2-furanmethanol 2-furanethanol, π-methoxy-(S)phenol, 2-methoxy-4-(1-propenyl)-, (Z)furan, tetrahydro-2,5-dimethoxylevoglucosan ethane, 1,1,1-trimethoxy2-furancarboxaldehyde, 5-(hydroxymethyl)1,4:3,6-dianhydro-πD-glucopyranose methyl-(2-hydoxy-3-ethoxy-benzyl)ether

formula

MW

C6H10O4 C10H12O2 C6H10O2 C6H6O2 C4H6O C5H4O2 C5H6O C4H8O2 C8H10 C6H8 C6H8O C6H6 C7H10O C6H8 C7H10O C9H12 C8H10 C7H8O2 C9H12 C10H16 C6H6O2 C5H8O C5H4O2 C5H8O2 C6H6O C7H8O C7H8O C6H8O2 C7H8O C6H8O C7H8O2 C5H6O2 C8H10O C6H10O3 C8H10O2 C2H4O2 C8H10 C7H8 C5H6O2 C6H8O C4H4O2 C4H8O2 C3H6O2 C10H14O2 C4H6O2 C4H6O2 C5H6N2O C9H12O2 C6H8O C5H10N2O C5H6O2 C7H10O3 C10H12O2 C6H12O3 C6H10O5 C5H12O3 C6H6O3 C6H8O4 C10H14O3

146 164 114 110 70 96 82 88 106 80 96 78 110 80 110 120 106 124 120 136 110 84 96 100 94 108 108 112 108 96 124 98 122 130 138 60 106 92 98 96 84 88 74 166 86 86 110 152 96 114 98 142 164 132 162 120 126 144 182

D

BO2

1.5 2.4 8.1 2.6 1.4 1.1 6.8 4.2 7.2 6.8 6.4 4.6 6.9 3.1 1.2 4.0 1.4 1.9 6.4 6.7 2.7 2.1 3.0 1.0 7.1 7.9 1.2 6.6 7.2 1.4 5.2 1.7 3.6 2.8 1.8 1.2 1.0 8.3 7.1

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

BO1

10−3 10−3 10−3 10−3 10−3 10−3 10−4 10−3 10−4 10−4 10−3 10−4 10−4 10−3 10−2 10−2 10−4 10−4 10−4 10−4 10−3 10−3 10−2 10−3 10−4 10−3 10−3 10−3 10−4 10−3 10−3 10−2 10−3 10−3 10−1 10−2 10−2 10−3 10−3

2.6 2.2 6.2 5.9 4.9 3.5 2.4 2.1 1.8 1.3 1.3 1.3 1.3 1.1 7.0 4.0 3.0 2.0 2.0 1.0 5.5 8.4 2.3 6.5 3.2 2.4 1.5 9.4 1.5 1.4 1.2 8.9 1.3 5.6 2.1 6.7 2.3 2.9 8.7 9.0 3.3 2.4 3.4 1.1 7.2 7.8 1.0 5.5 5.9 1.1 3.3 7.3 8.0 5.5

× × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−3 10−3 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−5 10−5 10−5 10−5 10−5 10−3 10−3 10−2 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−2 10−4 10−3 10−3 10−2 10−2 10−4 10−4 10−4 10−4 10−3 10−3 10−2 10−3 10−4 10−3 10−3 10−3 10−4 10−3 10−3 10−3 10−4 10−4

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Energy & Fuels Table 2. continued content, g/g bio-oil no.

compound

formula

MW

60 61 62 63 64 65 66 67 68 69 70 71

2-cyclopenten-1-one, 2-hydroxy2-propanone, 1-(acetyloxy)2-methoxy-4-vinylphenol ethanone, 1-(4-hydroxy-3-methoxyphenyl)hexanal dimethyl acetal 4-methyl-5H-furan-2-one 2-butene, 1,1-dimethoxy4-methoxypyridine-N-oxide 4-hydroxy-2-methoxycinnamaldehyde 2-propanone, 1-(4-hydroxy-3-methoxyphenyl)eugenol 2,5-dihydroxypropiophenone

C5H6O2 C5H8O3 C9H10O2 C9H10O3 C8H18O2 C5H6O2 C6H12O2 C6H7NO2 C10H10O3 C10H12O3 C10H12O2 C9H10O3

98 116 150 166 146 98 116 125 178 180 164 166

BO2 4.5 3.5 3.3 2.1 1.4 1.4 1.3 1.1 8.9 8.3 6.7 6.3

× × × × × × × × × × × ×

BO1

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−4 10−4 10−4 10−4

graphitized carbon. Another advantage of using FGMCNs is that the amorphous carbon derived from the LG reaction can be easily distinguished from the graphitized carbon by TGA. Figure 3 shows the liquid, solid, and gas yields from the reaction between LG and GMCNs at different temperatures. The solid here included that on the inner wall of the reactor system and on the surface of GMCNs. Overall, high temperature favored the conversion of LG to liquid and gas products. During all blank experiments, a significant amount of deposited LG was found at the bottom of the quartz tube (below the reaction zone), where the temperature was relatively low, leading to a high solid yield with 70−80 wt % for each blank run. This indicates that most of the LG sample was not decomposed at the reaction zone because of a short residence time. As GMCNs were introduced to the reaction zone, the LG vapor residence time was prolonged because of the GMCNs adsorption and the conversion of LG to small molecules was more significant. Figure 3a shows similar product yields from the reactions for HGMCN, CGMCN, and NFGMCN at 300 °C, which means that the temperature of 300 °C is not high

Figure 2. TGA curves of BO1-O, BO1-W, and BO2 under N2 flow.

mentioned above, this paper focuses on the interactions between LG and functional groups in char because of an insignificant ash content (0.1 wt % dry basis; see Table 1 in ref 19) in the pine sample studied here. FGMCNs with hydroxyl or carboxyl groups are employed to mimic the functional-groupcontaining char due to the chemical inertness of the parent

Figure 3. Product yields from the interactions between LG and GMCNs: (a) at 300 °C, (b) at 350 °C, (c) at 400 °C, and (d) at 450 °C. The solid weight was calculated by difference of the reactor and GMCNs before and after the experiments. In the case of 450 °C, the solid weight was corrected for the weight loss of GMCNs due to the removal of hydroxyl group (1.85 wt % of GMCNs) and carboxyl group (0.86 wt % of GMCNs; based on the TGA in Figure S2). E

DOI: 10.1021/acs.energyfuels.6b00725 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Table 3. Main Identified Compounds in the Liquid Product from the Interactions between LG and GMCNs at 400 °C relative abundance, % no.

name

formula

CGMCN

HGMCN

NFGMCN

blank run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

furfural levoglucosenone 1,4:3,6-Dianhydro-α-D-glucopyranose 2-furancarboxaldehyde, 5-methyl2-propanone, 1-hydroxy2-pentanone, 4-hydroxy-4-methylethanone, 1-(2-furanyl)furan, 2-propyl2(3H)-furanone 2-butanone, 1-(acetyloxy)2,5-dimethyl-4-hydroxy-3(2H)-furanone 2-propanone, 1-(acetyloxy)1,2-cyclopentanedione, 3-methyl2,3-anhydro-D-mannosan 2(3H)-furanone, 5-methyllevoglucosan 3-furaldehyde 2,3-pentanedione 2-cyclopenten-1-one, 2-methylfuran, 2,5-dimethyl1-hydroxy-2-butanone maltol 2(5H)-furanone furan, 3-methyl1,2-cyclopentanedione 4-methyl-5H-furan-2-one acetic acid, methyl ester 2-butanone toluene 1,3-dioxol-2-one 2H-pyran-2-one, 5,6-dihydro4-cyclopentene-1,3-dione

C5H4O2 C6H6O3 C6H8O4 C6H6O2 C3H6O2 C6H12O2 C6H6O2 C7H10O C4H4O2 C6H10O3 C6H8O3 C5H8O3 C6H8O2 C6H8O4 C5H6O2 C6H10O5 C5H4O2 C5H8O2 C6H8O C6H8O C4H8O2 C6H6O3 C4H4O2 C5H6O C5H6O2 C5H6O2 C3H6O2 C4H8O C7H8 C3H2O3 C5H6O2 C5H4O2

23.3

41.5

31.7

4.4 7.5

19.1

13.6

1.6 2.9 1.5

3.0 5.7 5.2

3.6 4.2 3.1 3.9

6.8 4.5 5.0

5.7

12.4

2.5 5.0 3.4 6.4 2.6

2.1

27.6 17.9 9.1 7.7 5.4 4.6 4.3 2.9 2.8 2.5 2.0 1.9 1.8 1.6 1.6 1.6 1.5

1.3 4.1 3.8 3.2 3.2 2.8 2.3 2.2 1.8 1.7 1.6 1.3 1.3

1.9 4.2

2.4

1.8 3.3 6.0

The hydroxyl group is probably removed at such relatively high temperature, leading to nearly equivalent product yields for the runs with HGMCN and NFGMCN. Nevertheless, the interactions between the carboxyl group and LG took place with a higher degree at 450 °C. It was the first time that the liquid yield with 40.3 wt % is higher than the solid yield (33.8 wt %) among all the runs. Compared with the runs for NFGMCN in Figure 3d,c, similar increments (10.1 and 11.5 wt %, respectively) of the liquid yields from CGMCN were detected, which suggests that carboxyl groups still worked well even at a temperature as high as 450 °C. The existence of the carboxyl group on GMCNs at 450 °C (0.42 wt %) is also supported by TGA under N2 flow (see Figure S2). The gas yield at 450 °C, 25.8 wt %, also reached an unprecedented level. Compared with the runs for CGMCN at 400 and 450 °C, higher temperature preferentially favored the conversion of LG to gas products. 3.4. Analyses of Liquid and Solid Products from the Interactions between LG and GMCNs. The liquid products from the interactions between LG and GMCNs were analyzed with GC/MS. Table 3 lists the main identified compounds and their relative abundance from all of the runs at 400 °C. The composition of the liquid product from the run with CGMCN is more complicated than those from other runs, implying that the carboxyl group promoted a greater conversion of LG to small molecules. Furfural is the most abundant compound

enough to facilitate the interactions between LG and the functional groups in GMCNs. However, the liquid and gas yields from the blank run are significantly lower than those for HGMCN, CGMCN, and NFGMCN because of a shorter LG vapor residence time, as mentioned above. The result from the reaction at 350 °C, as shown in Figure 3b, can be explained in the same manner, which indicates that the functional groups in GMCNs played an insignificant role at 350 °C. As the reaction temperature went up to 400 °C, the liquid yield dramatically increased from 26.3 wt % with NFGMCN to 35.9−37.8 wt % with FGMCN, as seen in Figure 3c. The interactions between functional groups in GMCNs and LG are thus confirmed. The liquid yield with CGMCN is slightly higher than that with HGMCN, indicating nearly equivalent effects of hydroxyl and carboxyl groups on the interactions between them and LG at 400 °C. The acidic property of the carboxyl group might have catalyzed the conversion of LG. The increase in the liquid yield by a longer residence time of LG vapor was 6.6 wt % based on the results from NFGMCN and the blank run, while the functional groups enabled the liquid yield to rise by 9.6 wt % (hydroxyl group) and 11.5 wt % (carboxyl group). Thus, the presence of the functional groups played a more important role than the influence of the residence time of LG vapor in the interactions between LG and FGMCNs. A similar, but less significant, trend of the changes in gas yield is also found in Figure 3c. Figure 3d shows the product yields from 450 °C. F

DOI: 10.1021/acs.energyfuels.6b00725 Energy Fuels XXXX, XXX, XXX−XXX

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formation of gas, liquid, and solid products as indicated by the GC, GC/MS, and TGA results, respectively. The reaction scheme of LG and CGMCN based on the experimental results is shown in Figure 5. The real volatiles in biomass pyrolysis may include different carbohydrates such as dehydrated sugars with a structure similar to LG, and the char is characterized by the developing structure and functional groups during the pyrolysis process. According to in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra, the carboxylic group may exist between 150 and 600 °C during carbonization of starch.31 Considering that the typical temperature for biomass pyrolysis is between 400 and 600 °C, the finding in the present study should have a general impact on all biomass pyrolysis processes. On the basis of the above results, the volatiles and char interaction could be very important for understanding the pyrolysis performance in different biomass pyrolysis reactors. The yield and property of bio-oil are highly dependent on the pyrolysis reactor. The pyrolysis reactor with a short residence time and high heat transfer, such as fluidized bed and cyclone reactor, has very high bio-oil yield but also contains a lot of HO. The pyrolysis reactor with stronger volatiles−char interactions, such as fixed bed and countercurrent moving bed reactor, produces less bio-oil but also less HO. A future direction in biomass pyrolysis should be the integration of high bio-oil yield and high LO percentage. A novel pyrolysis reactor, which can strengthen both bio-oil production and volatiles−char interactions, should be developed for this purpose.

regardless of whether GMCNs or functional groups existed, followed by 5-methyl-2-furancarboxaldehyde except the blank run, in which levoglucosenone was the second richer compound. However, levoglucosenone was absent from the runs with GMCNs, which was probably sensitive to the residence time. CGMCN and HGMCN before and after the runs at 400 °C were analyzed by TGA under an atmospheric flow of air. Figure 4 draws mass release profiles of the detected samples. It was

Figure 4. TGA curves of CGMCN and HGMCN before and after the runs at 400 °C under air flow.

clearly found that GMCNs after the runs were much easier to be combusted than those before the runs, which can be explained by the morphological feature of two different carbons. The carbon in GMCNs before the runs only existed in its graphitized state, while the LG-derived carbon at relatively low temperature on the GMCNs surface after the runs was characterized by an amorphous state, and the former was much more stable than the latter. 3.5. Understanding of Volatiles and Char Interaction by LG and GMCNs Reaction. As indicated in the Introduction, interaction between volatiles and char during biomass pyrolysis exists in all types of pyrolysis reactors. However, the investigation of such interaction is extremely difficult due to the ongoing development nature of both volatiles and char structure. In the above sections, the reaction between LG and GMCNs was studied in detail as the model reaction between volatiles and char. The results suggest that the functional group, especially the carboxylic group, has a large influence on the LG decomposition. The acidic nature of the carboxylic group may catalyze the reaction of LG or be involved in the reaction. The secondary reaction of LG leads to the

4. CONCLUSION The products obtained from biomass pyrolysis with and without HO recycling were analyzed in detail to understand the HO conversion. The results show that the recycled HO was completely converted to char, LO, and gas. In terms of the liquid products, the HO recycling favored the production of monoaromatic hydrocarbon, furans, and monooxygen-containing phenols, while it reduced or removed sugars and trioxygencontaining phenols. Particularly, LG, the most abundant compound with 0.18 g/g bio-oil, was fully converted during the HO recycling. The interactions between LG and GMCNs at 300, 350, 400, and 450 °C were further investigated. Both the residence time of LG vapor and the functional groups in GMCNs influenced the conversion of LG. The temperature significantly affected the interactions between LG and functional groups. The interactions between the hydroxyl group and LG took place only at 400 °C to yield 35.9 wt % of liquid and

Figure 5. Reaction scheme of LG vapor and CGMCN based on the experimental results. G

DOI: 10.1021/acs.energyfuels.6b00725 Energy Fuels XXXX, XXX, XXX−XXX

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9.1 wt % of gas, while the carboxyl group interacted with LG starting at 400 °C and intensifying at 450 °C with a liquid yield of 40.3 wt % and a gas yield of 25.8 wt %.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b00725. Temperature history of samples for producing BO1 and BO2; TGA curves of NFGMCN and CGMCN under N2 flow (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-i.H.). *E-mail: [email protected] (Y.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21506008). REFERENCES

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DOI: 10.1021/acs.energyfuels.6b00725 Energy Fuels XXXX, XXX, XXX−XXX